SUMMARY OF THE PRIOR ART
It has been common for many years to provide a base metal such as iron with a bright, non-corroding and permanently adhered metallic coating through wet mechanical plating processes. Such processes generally involve the utilization of metal powder particles that are cold welded to the base metal by impacting against its surfaces. The parts to be plated are placed in an agitated container with the metal powder particles, with or without promoter chemicals to aid in the plating process and with or without an impacting medium such as a mass of fine glass shot, and immersed in a liquid such as water. Agitation of the mass of particulate media, metal powder and metal parts through container agitation serves to provide the impacting force necessary to the plating operation while the liquid serves both as a carrier and a lubricant.
Generally, the parts to be plated are cleansed and copper flashed prior to the actual plating operation and plated parts are separated from the particulate impacting media and the plating liquid when the desired thickness of plating metal has been plated on the parts. Processes of this sort are disclosed, for instance, in U.S. Pat. Nos. 2,689,808 to Clayton and 3,531,315 to Golben. A more recent version of a mechanical plating process is described, for instance, in applicant's prior U.S. Pat. Nos. 3,690,935 and 3,776,186.
According to the prior art, a typical process for providing base metal articles with a metallic coating comprises a batch operation wherein the base metal articles to be plated are admixed with impacting media, i.e., a matrix material consisting of a mass of fine glass beads or similar discrete solid particles, and with a suitable inhibited acidic cleaner in a container comprising suitable means for imparting mechanical energy to its contents. For instance, the container may be a closed or an open-top barrel having a shape which can induce agitation when rotated, for example, a polygonal cross-section, or a round barrel with internal struts, which can be suitably agitated, e.g., by rotation around a horizontal or, in the case of an open-top barrel, around an inclined axis.
In the case of open top-barrels, it is advantageous to use an "oblique" or truncated cone construction, i.e., a configuration wherein the container has a wider cross-section at its solid base than at its open top such that the side walls form an angle of, for instance, 80.degree. with the base and on oblique angle with the top. When in operation, such a barrel is rotated around an inclined axis such that the plating load runs about one-half to two-thirds the way up the lower sidewall.
After several minutes of agitation and cleaning an aqueous solution of cupric sulfate and sulfuric acid is then added to this mixture. The resulting mixture is agitated until copper deposition takes place on the cleaned base metal as a pre-plate.
A solid, water-soluble promoter chemical such as a tin salt is then normally dissolved in this mixture and a quantity of plating metal in powder form, such as zinc, cadmium, brass, copper, aluminum, tin, gold, silver or the like, is added. A dispersant for the metal powder is also preferably included if a smooth plating is desired. In this manner tin plating, for instance, is first caused to form over the copper and fine particles of the plating metal such as zinc become loosely attached to the copper-tin coated parts. The plating metal becomes impact plated or uniformly cold welded on the article by the continuing action of the impacting media and mutual impacting of the metal parts on each other when agitation of the mixture is continued for a sufficient time.
The treating liquid is finally separated from the metal-coated articles and the impacting media. This liquid which contains unused metal powder in suspension, as well as dissolved zinc and/or cadmium, copper, tin and iron, is generally discarded. The metal-coated articles which have been separated from the impacting media are rinsed clean. Rinsing is also often employed between the cleansing and copper flashing stages and sometimes between the copper flashing and the principal plating stages.
One problem associated with such previously known metal surface treating processes is the pollution which is caused by the effluent streams containing unused particles of plating metals, acid and other chemicals suspended or dissolved in the process liquids which are discarded. Another problem heretofore associated with impact plating processes has been the great volume of fresh water required to conduct the required cleanings, rinsings, etc. Yet another problem has been the loss in the discarded effluents of all the chemical and metallic components which are added in excess during the plating cycle and which are not actually expended in the process but are required to make the process function properly for reasons of container volume, geometry and lubrication of the parts to be coated, etc.